1. Field of the Invention
The present invention relates to heating elements, and, more especially, to heating elements of a type comprising an electric resistor and a composite, electrically insulating substrate therefor.
2. Description of the Prior Art
It has long been known to the art to embed electric resistors within various polymeric materials. For example, French Pat. No. 796,138 describes electric resistors embedded within certain methacrylic polymers. This patent also describes a device wherein a resistor wire is wound into a plate or frame of synthetic material, which in turn is itself embedded in the same synthetic material, or in a different synthetic material. This technique makes it possible to avoid using such massive heating equipment wherein an unprotected electric resistor is exposed to ambient conditions, and which merely is borne by any suitable support. But taking into account the resins available at the time of the filing of this French patent, it is obvious that such heating elements could not be brought to a high temperature without causing the decomposition, or thermal degradation, of the polymer comprising the same. And as soon as one effected a reduction in working temperatures, it logically followed that it was not possible to produce either heating elements having a sufficiently high heating power per unit surface, or radiant heating elements. The term radiant heating element of course denotes any heating element which can effect the transfer of heat through rays or radiation. This particular method of heating is quite useful and highly advantageous in certain applications, especially where it is desired to obtain rapid and localized heating with an installation of but limited power. With respect to the construction or fabrication of heating elements having a high power per unit surface, difficult technical problems arise, namely firstly, if a large number of electric resistor wires are mounted in the heating element, or if such wires are not arranged in an exact and uniform pattern, there is a great risk that such wires may come into contact with one another and cause partial short circuits, with all attendant consequences; secondly, if the electric resistor wires are not suitably coated with resin, the heat produced by the wires is but poorly transmitted and there is a risk of overheating of the wires, which, next giving use to excessive temperatures, favors local thermal degradation of the resin; thirdly, if such heating elements are used in applications such as electric household appliances, in which the user has no especial training, it is necessary that the heating elements be capable of being used with marked safety, and some government or other standards even direct that the heating element should withstand without damage the direct action of a stream of water; fourthly, if the amount of resin in which the electric resistor wires are buried is too large, the heating elements may become too expensive; and fifthly, on the other hand, the amount of resin in which the electric resistor wires are buried is too small, or the electric resistor wires are improperly arranged there is a corresponding risk that the heat produced may be poorly distributed over the surface of the heating element, which would be harmful for certain applications, as well as to the resin comprising heating element.
Therefore, it is indeed quite difficult to produce acceptable heating elements having a high power per unit surface, and it is accordingly trivially apparent that, in order to produce same, one could not simply avail oneself of the teachings of the aforesaid French Pat. No. 796,138 by simply replacing the resins of that day with today's more thermally stable resins.
Developments contributing to the state of the art, subsequent to that described in the noted French Pat. No. 796,138, include:
That disclosed in the published German patent application, No. 2,346,648, i.e., a device in which electric resistor wires, arranged in parallel array, are embedded under pressure in a mixture of phenolic resin and either sawdust or wood chips; the structure of such device, however, does not display the properties required for fabrication of a good radiant heating element, or one yielding high power per unit surface.
Also, in published German patent application No. 2,357,727, there is described a pliable mat composed of heat conductors embedded in an insulating material and covered with a sheet of aluminum foil; but the purpose of such a device is simply to make possible the defrosting of food and other dishes kept at a very low temperature. It is thus quite obvious that such a device is as remote as possible from useful radiant heating elements or from heating elements yielding high power per unit surface.
And French Pat. No. 1,490,850 discloses flexible electric heating elements, of the fabric, or wire or cord type, but, as a result of their very nature, these are heating elements which are not self-sustaining. In many applications, therefore, such elements must be complemented by reinforcement or suitable support, or even be attached to the object sought to be heated.
The focus of French Pat. No. 2,158,258 are heating elements desired to equip structures or containers in which the heating element is secured contiguous the surface of the particular structure under consideration. For this purpose, a stratified preparation impregnated with certain polyimides in the form of pre-polymers is prepared, and thence the polymerization is completed in situ, when the stratified preparation is already installed on the structure sought to be heated. It is apparent that this method of construction is practicable only when it is possible or feasible to permanently connect the heating element to the object to be heated, and only when the latter can be heated by direct conduction; accordingly, such patented invention can be utilized for but a limited number of applications.
Compare also the French Patent of Addition No. 2,305,088, available to the public as of October, 1976, and wherein are described radiation heating elements which include a support based on a thermostable resin (for example, polyimide), transparent to infrared radiation, and silica-based fibers, on which support is mounted an electric resistor circuit, in the standard manner of printed circuits, on a thin layer (a few microns), and the entire assembly is coated with an insulating varnish, such as silicone, and with a metallic reflecting layer serving as a reflector. Such a device nonetheless manifests a number of drawbacks; firstly as a result of its thinness, the electric resistor circuit tends to become oxidized and then, therefore, to break (especially when made from copper or silver); secondly when made from metals which are difficult to oxidize, this type of electric resistor circuit requires techniques poorly suited to industrial-scale production for its manufacture, which makes them expensive; thirdly the electric resistor circuit usually includes a profile with projections, which has a deleterious effect on the quality of electrical insulation and on the effectiveness of the performance of the silicone varnish (risk of cracking as a result of point effect); and fourthly the latter drawback is even more emphasized as the metal reflector has a definite tendency to produce short circuits with the electric resistor circuits.